On the Feature Codes configuration page, go to the Core section and you should find there approximately in the middle In-Call Asterisk Toggle Call Recording - the default value is *1. While on call, pressing *1 will toggle the recording state.
Make sure that adhoc recording is allowed for the call type (incoming/outgoing/caller/callee) - that is done by adding w or W on the General Settings page in the field Outbound Trunk Calls and Other Calls.